Physically defined varactor in a CMOS process

ABSTRACT

A differential varactor is physically defined in a CMOS process using a using the diffusion mask of a polycide gate rather than a P (+) mask, as is commonly used. The differential CMOS varactor may be used in a a phase locked loop (PLL) of a voltage-controlled oscillator (VCO) to enable a transceiver to communicate at OC-3/STM-1 data rates using SONET/SDH signaling formats.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is related to integrated circuits and, inparticular, to a variable capacitor in complementary metal oxidesemiconductor (CMOS) process.

2. Background Information

Data access rates for Synchronous Optical Network (SONET) andSynchronous Digital Hierarchy (SDH) networks are steadily increasing asdemand for web content move toward video-on-demand, streaming video, andaudio, and teleconferencing applications. The current trend is toprovide such high bandwidth access over coaxial cables and opticalfibers has using SONET and SDH signaling formats. SONET specifies a setof transmission speeds (or data rates), which are multiples of theoptical carrier level one (OC-1) channel data rate of 51.840 megabitsper second (Mbps). For example, it is common to provide access atOptical Carrier level three/Synchronous Transfer Module 1 (OC-3)/(STM-1)rate, which is a data rate of 155.52 megabits per second (Mbps).

To accomplish data access at such data rates, the SONET/SDH standardsdemand components that have high levels of integrations, low powerconsumption, and low jitter. Jitter is commonly defined as short-termvariations of a digital signal's significant instants from their idealpositions in time. Jitter tolerance is the peak-to-peak amplitude ofsinusoidal jitter applied at the line interface input that causes anequivalent 1 dB signal-to-noise ratio (SNR) loss measured as bit errorrate (BER)=10⁻¹⁰. The stringent SONET specify that the jitter gain byoptical transceivers on an OC-3 channel (155.52 Mbps) be less than 0.1dB and that the output jitter (e.g., jitter generation, intrinsicjitter) be less than 0.1UI peak-to-peak or 0.01UI rms

Such demanding standards may be difficult to achieve with currenttransceivers implemented in complementary metal oxide semiconductor(CMOS) technology. To explain, an oscillator with a low jitter gain isusually an inductor-capacitor tank voltage controlled oscillator (LCtank VCO).

What limits the noise in an LC tank is its small quality factor (Q),which is a measure of the LC tank's frequency response (i.e., it's noisebandwidth). A degraded Q causes the center frequency of the LC tank toshift and it'ts output jitter to increase, as is well known. The Q of anLC tank is given by:

Q=ωL/R _(eff)=1/(ωC _(eq))(R _(eff))  (Equation 1)

where ω is the angular frequency of the signal through the capacitor andinductor, C_(eq) is the equivalent capacitance in farads, L is the valueof the inductor in henrys, and R_(eff) is the equivalent resistance (orresistivity) of the circuit.

To obtain a high Q, the inductance (L) may be increased, the capacitance(C) may be decreased, and the effective resistance (R_(eff)) may bedecreased. It is not anticipated the any technological advances willimprove inductance-related Q. Therefore, to improve LC tank Q, thecapacitance-related Q may be matched to the L-related Q using avoltage-controlled capacitor (or varactor). In low frequencyapplications, the inductor Q is typically around four or five. Prior artCMOS voltage controlled capacitors have similar Qs in low frequencyapplications (e.g., around five or six). In high frequency applications,the inductor Q can be as high as twenty or thirty. Unfortunately, priorart CMOS voltage controlled capacitors do not have comparable Qs becauseof relatively large parasitic resistance (R_(eff)). In many instances,the Q of the LC tank is so degraded that it may be unusable in SONET orother high-speed environments having demanding jitter performancestandards because the parasitic resistance is a source of noise, whichreduces jitter performance.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, like reference numbers generally indicate identical,functionally similar, and/or structurally equivalent elements. Thedrawing in which an element first appears is indicated by the leftmostdigit(s) in the reference number, in which:

FIG. 1 is a perspective view of a fragment of a CMOS differentialvaractor 100 according to embodiments of the present invention;

FIG. 2 is a schematic diagram of an equivalent circuit 200 of a portionof the CMOS varactor 100 having spacing between the two rectangulardiffusions of P(+) material (102, 104) no less than approximately 0.5microns;

FIG. 3 is a schematic diagram of an equivalent circuit 300 of a portionof the CMOS varactor 100 according to an embodiment of the presentinvention;

FIG. 4 is a high-level block diagram of CMOS transceiver according toembodiments of the present invention;

FIG. 5 is a high-level block diagram of a communications systemaccording to embodiments of the present invention; and

FIG. 6 is a flowchart of a fabrication process to fabricate a CMOSvaractor according to embodiments of the present invention.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

A CMOS varactor according to embodiments of the present invention isdescribed herein. In the following description, numerous specificdetails, such as particular processes, materials, devices, and so forth,are presented to provide a thorough understanding of embodiments of theinvention. One skilled in the relevant art will recognize, however, thatthe invention can be practiced without one or more of the specificdetails, or with other methods, components, etc. In other instances,well-known structures or operations are not shown or described in detailto avoid obscuring various embodiments of the invention.

Some parts of the description will be presented using terms such astransceiver, phase-locked loop, substrate, resistance, capacitance,jitter, and so forth. These terms are commonly employed by those skilledin the art to convey the substance of their work to others skilled inthe art.

Other parts of the description will be presented in terms of operationsperformed by a computer system, using terms such as accessing,determining, counting, transmitting, and so forth. As is well understoodby those skilled in the art, these quantities and operations take theform of electrical, magnetic, or optical signals capable of beingstored, transferred, combined, and otherwise manipulated throughmechanical and electrical components of a computer system; and the term“computer system” includes general purpose as well as special purposedata processing machines, systems, and the like, that are standalone,adjunct or embedded.

Various operations will be described as multiple discrete blocksperformed in turn in a manner that is most helpful in understanding theinvention. However, the order in which they are described should not beconstrued to imply that these operations are necessarily order dependentor that the operations be performed in the order in which the blocks arepresented.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure, process, block,or characteristic described in connection with the embodiment isincluded in at least one embodiment of the present invention. Thus, theappearances of the phrases “in one embodiment” or “in an embodiment” invarious places throughout this specification are not necessarily allreferring to the same embodiment. Furthermore, the particular features,structures, or characteristics may be combined in any suitable manner inone or more embodiments.

FIG. 1 is a perspective view of a fragment of a CMOS differentialvaractor 100 according to embodiments of the present invention. In oneembodiment of the present invention, the varactor 100 may be a P-Njunction diode formed from source-drain diffusions of a p-type metaloxide semiconductor field effect transistor (p-MOSFET) within an n-well.

For example, the varactor 100 includes a P-N diode formed using tworectangular diffusions of P (+) material (102, 104) inside an n-well108. An N (+) well contact 106 is formed inside the n-well 108. Then-well 108 is formed in a (p) substrate 110 (e.g., epitaxial substrate,non-epitaxial substrate, silicon-on-insulator (SOI) substrate,silicon-on sapphire (SOP) substrate). The N (+) well contact 106 iselectrically connected (electrical connection 114) to the two P (+)diffusions 102, 104. A control voltage 112 is applied to move the N (+)well contact 106 up and down inside the n-well 108 to vary the depletionregions between each P (+) diffusions (102, 104) and the n-well 108,which changes the capacitance between each of the two P (+) diffusions102, 104 and the n-well 108.

Because the two P (+) diffusions 102, 104 are operated like adifferential capacitance in that when the voltage on the P (+) material102 is increasing, the voltage on the P (+) material 104 is decreasing,and vice versa, the closer the two rectangular diffusions of P (+)material (102, 104) are to each other, the better the coupling betweenthem. The diffusion masks commonly used to locate the two rectangulardiffusions of P (+) material (102, 104) typically allow spacing betweenthe two rectangular diffusions of P (+) material (102, 104) no less thanapproximately 0.5 microns, however.

To illustrate, FIG. 2 is a schematic diagram of an equivalent circuit200 of a portion of the CMOS varactor 100 as described so far (i.e.having a spacing between the two rectangular diffusions of P(+) material(102, 104) no less than approximately 0.5 microns). The equivalentcircuit 200 shows a capacitance 202 and a resistance 204 coupled inseries with the P (+) material 102, and a capacitance 206 and aresistance 208 coupled in series with the P (+) material 104. Both legs(i.e., capacitance 202/resistance 204 and capacitance 206/resistance208) are coupled in parallel with a series resistance 210 to N (+) wellcontact 106.

The series resistances 208 and 208 are parasitic resistances, however,and reduce the Q of the differential capacitance created by the two P(+) diffusions 102, 104. As the Q of the differential capacitancecreated by the two P (+) diffusions 102, 104 degrades, the jitter in anyLC tank using the varactor 100 increases. The resistance 210 has anegligible effect on the Q. The closer the two rectangular diffusions ofP (+) material 102, 104 are to each other, the smaller the resistances204 and 208 become. However, as described above, current diffusion maskslimit the spacing between the two rectangular diffusions of P (+)material 102, 104.

According to an embodiment of the present invention, the spacing betweenthe two rectangular diffusions of P (+) material 102, 104 is reduced byusing the diffusion mask of a polycide gate 116 rather than a P (+)mask, as is commonly used. For example, the spacing between the tworectangular diffusions of P (+) material 102, 104 may be reduced to lessthan approximately 0.3 microns using a 0.35-micron CMOS process. Assuch, the resistances from the two diffusions of P (+) material 102, 104back to the N (+) well contact 106 is reduced such that it becomesnegligible, something not possible with conventional diffusion maskingtechniques.

To illustrate, FIG. 3 is a schematic diagram of an equivalent circuit300 of a portion of the CMOS varactor 100 according to an embodiment ofthe present invention (e.g., including the gate 116). The equivalentcircuit 300 shows a capacitance 302 and a resistance 304 coupled inseries with the P (+) material 102 and the P (+) material 104,respectively. Both the capacitance 302 and the capacitance 304 arecoupled to the series resistance 210 to N (+) well contact 106. Notethat the parasitic resistances are negligible.

Although embodiments of the present invention are described with respectto a varactor formed from the source-drain diffusions of a p-MOSFET(e.g., the two P (+) diffusions (102, 104)) formed within an n-well(e.g., the n-well 108), embodiments of the present invention also applyto an n-MOSFET formed within a P (+well).

In alternative embodiments of the present invention, the spacing betweenthe two rectangular diffusions of P (+) material 102, 104 may be furtherreduced by using lightly-doped drain (LDD) structures, using haloimplant, and/or using LDD with halo implant. Such uses may cause the tworectangular diffusions of P (+) material 102, 104 to extend under thegate 116 edges.

FIG. 4 is a schematic block diagram of a phase-locked loop 400 accordingto embodiments of the present invention. The phase-locked loop 400includes a voltage-controlled oscillator (VCO) core 402, which outputsclock pulses 404 to an optional clock divider 406. In embodiments of thepresent invention having a clock divider, the clock divider 406 maydivide the clock pulses 404 to lower frequency clock pulses 408, whichare input to a phase detector 410. Alternatively, the division ratio maybe “one” and the phase-locked loop 400 has no clock divider. The phasedetector 410 drives a charge pump 412, which drives a loop filter 414.The loop filter 414 drives a buffer 416, which drives thevoltage-controlled oscillator core 402 to output the clock pulses 404.

The VCO core 402 includes a CMOS varactor 420, which is represented by ap-MOSFET whose gate (116) is coupled V_(DD) and whose substrate (110) iscoupled to the control voltage 112 (e.g., as supplied by the phasedetector 410 through the buffer 416). The CMOS varactor 420 may be anyCMOS varactor implemented according to embodiments of the presentinvention.

The VCO core 420 also includes a pair of inductors 422,424, which may beformed in the same substrate (110) as CMOS varactor 420. Inductorssuitable for implementing the pair of inductors 422, 424 are well known.The VCO core 420 also includes a pair of n-MOSFETS 426, 428. MOSFETSsuitable for implementing the n-MOSFETS 426, 428 are well known.

According to an embodiment of the present invention, thevoltage-controlled oscillator core 402 may be running at greater thanone GHz and can be running at approximately 1.25 GHz.

Dividers suitable for implementing the clock divider 406 are well known.

Phase detectors suitable for implementing the phase detector 410 arewell known.

Charge pumps suitable for implementing the charge pump 412 are wellknown.

The loop filter 414 includes a resistor 430 and a pair of capacitors432, 434. Resistors and capacitors suitable for implementing the loopfilter 414 are well known.

FIG. 5 is a high-level block diagram of a communications system 500according to embodiments of the present invention. The system 500includes a transceiver 502, which may be a front-end transceiversuitable for 155 Mbps OC3/STM1/Asynchronous Transfer Mode (ATM)transmission applications. The example transceiver 502 includes a pairof phase-locked loops 504 implemented according to embodiments of thepresent invention.

The example transceiver 502 interfaces to one or more well-known orfuture fiber optic modules 506 and/or coaxial transformers 508 on theline side, and to a well-known or future SONET/SDH overhead terminator510 or a well-known or future ATM User Network Interface (UNI) 512 onthe system side. The example transceiver 502 may include a well-known orfuture microprocessor 514, which may provide software-mode control ofthe transceiver 502 in any suitable manner.

The system 500 and other devices implemented according to embodiments ofthe present invention may be suitable for use as/in opticalcross-connects (OXC), optical add/drop multiplexers (OADM) that operateat the optical carrier level 3 (OC-3), short haul serial links, accesslinks for Asynchronous Transfer Mode (ATM) wide area networks (WAN),digital loop carriers which has a data rate of approximately 155.52Mbps. Alternative embodiments of the present invention may be suitablefor use as/in optical cross-connects (OXC) and/or optical add/dropmultiplexers (OADM) that operate at OC-768, which has a data rate of39.81312 Gbps. Of course, other SONET/SDH data rates are suitableincluding 155 Mbps, 622 Mbps, 2.488 Gbps, 9.953 Gbps, and 39.8 Gbps)

FIG. 6 is a flowchart of a fabrication process 600 to fabricate a CMOSvaractor according to embodiments of the present invention. Amachine-readable medium having machine-readable instructions thereon maybe used to cause a processor to perform the process 600. Of course, theprocess 600 is only an example process and other processes may be used.

In a block 602, an n-well is formed in or on a p-substrate usingwell-known or future techniques. According to embodiments of the presentinvention, the n-well is diffused into the p-substrate using well-knownor future diffusion techniques.

In a block 604, two rectangular diffusions of P (+) material are formedin or on the n-well and a polycide gate is used to provide the spacingbetween the two rectangular diffusions of P (+) material. A polycidegate mask may be used to define the spacing between the two rectangulardiffusions of P (+) material. An LDD structure, a halo implant, and/oran LDD with a halo implant may further define the spacing between thetwo rectangular diffusions of P (+) material. Although halo implants arecommonly used to, among other things control leakage currents, inembodiments of the present invention, however, the halo implants areused to define the spacing between the two rectangular diffusions of P(+) material.

According to embodiments, a metal oxide structure is built on top of then-well diffusion and the two rectangular diffusions of P (+) materialare laid out (e.g., etched) as a P-N diode using well-known or futuretechniques. In one embodiment of the present invention, a layer of metaloxide is deposited using well known or proprietary plasma enhancedchemical vapor deposition (PECVD) techniques, for example. In otherembodiments of the present invention, forms a semiconductor diode on theSOI substrate. According to embodiments of the present invention, theP-N diode is formed by doping portions of the n-well with P-typematerial in a well-known manner using ion implantation, diffusion fromspin-on solutions, or other current or future techniques

In a block 606, a diffusion of N (+) material is formed in or on then-well to create an N (+) well. According to embodiments, N (+) materialis diffused into the n-well using well-known or future techniques.

Although embodiments of the present invention are described with respectto a voltage variable capacitor in an LC-type resonator circuit,embodiments of the present invention are not so limited. For example,embodiments of the present invention may include self-balancing bridgecircuits, parametric amplifiers, filters, or other suitable circuits.

Embodiments of the invention can be implemented using hardware,software, or a combination of hardware and software. Suchimplementations include state machines and application specificintegrated circuits (ASICs). In implementations using software, thesoftware may be stored on a computer program product (such as an opticaldisk, a magnetic disk, a floppy disk, etc.) or a program storage device(such as an optical disk drive, a magnetic disk drive, a floppy diskdrive, etc.).

The above description of illustrated embodiments of the invention is notintended to be exhaustive or to limit the invention to the precise formsdisclosed. While specific embodiments of, and examples for, theinvention are described herein for illustrative purposes, variousequivalent modifications are possible within the scope of the invention,as those skilled in the relevant art will recognize. These modificationscan be made to the invention in light of the above detailed description.

The terms used in the following claims should not be construed to limitthe invention to the specific embodiments disclosed in the specificationand the claims. Rather, the scope of the invention is to be determinedentirely by the following claims, which are to be construed inaccordance with established doctrines of claim interpretation.

What is claimed is:
 1. A method, comprising: disposing two rectangulardiffusions of P (+) material in an n-well formed in a p-substrate usinga complementary metal oxide semiconductor (CMOS) process; disposing apolycide gate between the two rectangular diffusions of P (+) material;disposing a pair of inductors on the substrate; and coupling the tworectangular diffusions of P (+) material and the pair of inductors in avoltage-controlled oscillator (VCO) configuration.
 2. The method ofclaim 1 wherein disposing two rectangular diffusions of P (+) materialin an n-well formed in a p-substrate using the CMOS process comprisesdisposing two rectangular diffusions of P (+) material in an n-wellformed in an epitaxial substrate using the CMOS process.
 3. The methodof claim 1 wherein disposing two rectangular diffusions of P (+)material in an n-well formed in a p-substrate using the CMOS processcomprises disposing two rectangular diffusions of P (+) material in ann-well formed in a non-epitaxial substrate using the CMOS process. 4.The method of claim 3 wherein disposing two rectangular diffusions of P(+) material in an n-well formed in a p-substrate using the CMOS processcomprises disposing two rectangular diffusions of P (+) material in ann-well diffused in a p-substrate using the CMOS process.
 5. The methodof claim 1 wherein disposing two rectangular diffusions of P (+)material in an n-well formed in a p-substrate using the CMOS processcomprises building a metal oxide structure on top of the n-well.
 6. Themethod of claim 1 further comprising defining the spacing between thetwo rectangular diffusions of P (+) material using a lightly doped drain(LDD) structure.
 7. The method of claim 1 further comprising definingthe spacing between the two rectangular diffusions of P (+) materialusing halo implantation.